The menstrual cycle ( or period) is the recurring physiological changes in a female's body that are under the control of the reproductive hormone system and necessary for reproduction. In women, menstrual cycles occur typically on a monthly basis between puberty and menopause. Besides humans, only great apes exhibit menstrual cycles, in contrast to the estrus cycle of most mammalian species.

During the menstrual cycle, the sexually mature female body releases one egg (or occasionally two, which might result in non-identical twins) at the time of ovulation. The lining of the uterus, the endometrium, builds up in a synchronised fashion. After ovulation, this lining changes to prepare for potential implantation of the fertilized egg to establish a pregnancy. If fertilisation and pregnancy do not ensue, the uterus sheds the lining and a new menstrual cycle begins. The process of the shedding of the lining is called menstruation. Menstruation manifests itself to the outer world in the form of the menses ( also menstruum): essentially part of the endometrium and blood products that pass out of the body through the vagina. Although this is commonly referred to as blood, it differs in composition from venous blood.

Common usage refers to menstruation and menses as a period. This bleeding serves as a sign that pregnancy has not taken place. (However, this cannot be taken as certainty, as sometimes there is some bleeding in early pregnancy.) During the reproductive years, failure to menstruate may provide the first indication to a woman that she may have become pregnant. A woman might say that her "period is late" when an expected menstruation has not started and she might have become pregnant.

Menstruation forms a normal part of a natural cyclic process occurring in healthy women between puberty and the end of the reproductive years. The onset of menstruation, known as menarche, occurs at an average age of 12, but can occur any time between the ages of 8 and 16.^[1] The last period, menopause, usually occurs between the ages of 45 and 55. Deviations from this pattern deserve medical attention. Amenorrhea refers to a prolonged absence of menses during the reproductive years of a woman for reasons other than pregnancy. For example, women with very low body fat, such as athletes, may cease to menstruate. The presence of menstruation does not prove that ovulation took place, and women who do not ovulate may have menstrual cycles. Those anovulatory cycles tend to take place less regularly and show greater variation in cycle length.

Women show considerable variation in the lengths of their menstrual cycle, and the length of the menstrual cycle differs in different animals (see below). While cycle length may vary, 28 days is generally taken as representative of the normal ovulatory cycle in women. Convention uses the onset of menstrual bleeding to mark the beginning of the cycle, so the first day of bleeding is called "Cycle Day one". One can divide the menstrual cycle into four phases:

Menstruation lasts for a few days (usually 3 to 5 days, but anywhere from 2 to 7 days is considered normal^[2]) and involves the loss of about 50 millilitres of blood (including shed lining). An enzyme called plasmin — contained in the endometrium — inhibits the blood from clotting. Because of this blood loss, women have higher dietary requirements for iron than do males — to prevent iron deficiency. Many women experience uterine cramps, also referred to as dysmenorrhea, during this time. A vast industry has grown to provide sanitary products to help women to manage their menses.

Through the influence of a rise in Follicle stimulating hormone (FSH), five to seven tertiary-stage ovarian follicles are recruited for entry into the menstrual cycle. These follicles, that have been growing for the better part of a year in a process known as folliculogenesis, compete with each other for dominance. In a signal cascade kicked off by luteinizing hormone (LH), the follicles secrete estradiol, a steroid that acts to inhibit pituitary secretion of FSH. With diminished FSH supply comes a slowing in growth that eventually leads to follicle death, known as atresia. The largest follicle secretes inhibin that serves as a finishing blow to less competent follicles by further suppressing FSH. This dominant follicle continues growing, forms a bulge near the surface of the ovary, and soon becomes competent to ovulate.

The follicles also secrete estrogens (of which estradiol is a member). Estrogens initiate the formation of a new layer of endometrium in the uterus, histologically identified as the proliferative endometrium. If fertilized, the egg cum embryo will implant itself within this hospitable flesh.

When the follicle has matured, it secretes enough estradiol to trigger the acute release of luteinizing hormone (LH). In the 28-day cycle this LH surge starts around cycle day 12 and may last 48 hours. The release of LH matures the egg and weakens the wall of the follicle in the ovary. This process leads to ovulation: the release of the now mature ovum, the largest cell of the body (with a diameter of about 0.5 mm). Which of the two ovaries — left or right — ovulates appears essentially random; no known left/right co-ordination exists. The Fallopian tube needs to capture the egg and provide the site for fertilization. A characteristic clear and stringy mucus exhibiting spinnbarkeit develops at the cervix, ready to accept sperm from intercourse. In some women, ovulation features a characteristic pain called Mittelschmerz which lasts for several hours. Many women perceive the vaginal and cervical mucus changes at ovulation, particularly if they are monitoring themselves for signs of fertility. An unfertilised egg will eventually disintegrate or dissolve in the uterus.

After ovulation, the residual follicle transforms into the corpus luteum under the support of the pituitary homones. This corpus luteum will produce progesterone in addition to estrogens for the next 2 weeks. Progesterone plays a vital role in converting the proliferative endometrium into a secretory lining receptive for implantation and supportive of the early pregnancy. It raises the body temperature slightly, thus women who record their temperature on a daily basis will notice that they have entered the luteal phase. If fertilisation of an egg has occurred, it will travel as an early embryo through the tube to the uterine cavity and implant itself about 5 to 7 days after ovulation. Shortly after implantation, the growing embryo will signal its existence to the maternal system. One very early signal consists of human chorionic gonadotropin (hCG), a hormone that pregnancy tests can measure. This signal has an important role in maintaining the corpus luteum and enabling it to continue to produce progesterone. In the absence of a pregnancy and without hCG, the corpus luteum demises and inhibin and progesterone levels fall. This will set the stage for the next cycle. Progesterone withdrawal leads to menstrual shedding (progesterone withdrawal bleeding), and falling inhibin levels allow FSH levels to rise to raise a new crop of follicles.

In many women, various unpleasant symptoms caused by the involved hormones and by cramping of the uterus can precede or accompany menstruation. More severe symptoms may include significant menstrual pain (dysmenorrhea), abdominal pain, migraine headaches, depression and irritability. Some women encounter premenstrual stress syndrome (PMS or premenstrual syndrome), a cyclic clinical entity. The list of symptoms experienced varies from person to person. Furthermore, within an individual, the severity of the symptoms may vary from cycle to cycle.

While the length of the follicular phase — and consequently the length of the menstrual cycle — may vary, the luteal phase almost always takes 14 days. Sperm can survive for 3 to 4 days (possibly up to 7 days) inside a woman, so the most fertile period (the time with the highest likelihood of sexual intercourse leading to pregnancy) covers the time from some 5 days before ovulation until 1–2 days after ovulation. In a normal, four-week cycle, this corresponds to the second and the beginning of the third week of the cycle. Various natural family planning methods of birth control attempt to determine the precise time of ovulation in order to find the relatively fertile and the relatively infertile days in the cycle.

People who have heard about the menstrual cycle and ovulation may commonly and mistakenly assume, for contraceptive purposes, that menstrual cycles always take a regular 28 days, and that ovulation always occurs 14 days after beginning of the menses. This assumption may lead to unintended pregnancies. Note too that not every bleeding event counts as a menstruation, and this can mislead people in their calculation of the fertile window.

If a woman wants to conceive, the most fertile time occurs between 16 and 14 days prior to the expected menses. Many women use ovulation detection kits that detect the presence of the LH surge in the urine to indicate the most fertile time. Other ovulation detection systems rely on the subtle temperature shift that progesterone induces.

Among women living closely together, the onsets of menstruation tend to synchronize somewhat. Researchers first described this phenomenon in 1971, and explained it by the action of pheromones in 1998 (Stern and McClintock 1998).

Extreme intricacies regulate the menstrual cycle. For many years, researchers have argued over which regulatory system has ultimate control: the hypothalamus, the pituitary, or the ovary with its growing follicle; but all three systems have to interact. In any scenario, the growing follicle has a critical role: it matures the lining, provides the appropriate feed-back to the hypothalamus and pituitary, and modifies the mucus changes at the cervix. Two sex hormones play a role in the control of the menstrual cycle: estradiol and progesterone. While estrogen peaks twice, during follicular growth and during the luteal phase, progesterone remains virtually absent prior to ovulation, but becomes critical in the luteal phase and during pregnancy. Many tests for ovulation check for the presence of progesterone. These sex hormones come under the influence of the pituitary gland, and both FSH and LH play necessary roles. FSH stimulates immature follicles in the ovaries to grow. LH triggers ovulation. The gonadotropin-releasing hormone of the hypothalamus controls the pituitary, yet both the pituitary and the hypothalamus receive feed-back from the follicle. After ovulation the corpus luteum — which develops from the burst follicle and remains in the ovary — secretes both estradiol and progesterone. Only if pregnancy occurs do hormones appear in order to suspend the menstrual cycle, while production of estradiol and progesterone continues. Abnormal hormonal regulation leads to disturbance in the menstrual cycle.

Some women with neurological conditions experience increased activity of their conditions at about the same time every month. 80 percent of women with epilepsy have more seizures than usual in the phase of their cycle when progresterone declines and estrogen increases.

During times when progesterone is highest, nerve-cell receptor subtype delta was high. Nerve cells with more delta receptors are less likely to fire than cells with lower numbers of delta receptors. When estrogen levels are higher, the number of delta receptors decrease, increasing nerve cell activity, in turn increasing anxiety and seizure susceptibility. (Maguire, et. al., 2005)

Unlike the case in other species, ovulation remains hidden in the human. That a woman may sense her own ovulation while it may remain indiscernible to others has sociobiological significance. In contrast, other species often signal receptivity. In this context, note that evidence suggests that women's preferences for men may change during their most fertile days, that is, before and shortly after ovulation. During this time, they may prefer different male scents, more masculine faces, and social presence in males considered as partners. (Gangestad 2004; debated)

Evidence suggests that eggs are formed from germ cells early in fetal life. The number is reduced to an estimated 400,000 to 450,000 immature eggs residing in each ovary at puberty. The menstrual cycle, as a biologic event, allows for ovulation of one egg typically each month. Thus over her lifetime a woman will ovulate approximately 400 to 450 times. All the other eggs dissolve by a process called atresia. As a woman's total egg supply is formed in fetal life, to be ovulated decades later, it has been suggested that this long life-time may make the chromatin of eggs more vulnerable to division problems, breakage, and mutation than the chromatin of sperm, which are produced continuously during a man's reproductive life. This possibility is supported by the observation that conceptuses and infants of older mothers have higher rates of chromosome abnormalities than those of older fathers.

Not all menstruations result from an ovulatory menstrual cycle. In some women, follicular development may start but not complete, nevertheless estrogens will form and will stimulate the uterine lining. Sooner or later the uterus will shed this lining. As no ovulation and no progesterone involvement occurs, doctors call this type of bleeding an estrogen breakthrough bleeding, and cannot always predict its duration or frequency. Anovulatory bleeding commonly occurs prior to menopause (premenopause) or in women with polycystic ovary syndrome.

The "normal menstrual cycle" occurs every 28 days ± 7 days. Doctors term cycles with intervals of 21 days or fewer as polymenorrhea and, on the other hand, call cycles with intervals exceeding 35 days oligomenorrhea (or amenorrhea if intervals exceed 180 days).

The normal menstrual flow amounts to 50 ml ± 30 ml. Flow in excess of 80 ml (hypermenorrhea or menorrhagia) may stem from hormonal disturbance, uterine abnormalities, including uterine leiomyoma or cancer, and other causes. Doctors call the opposite phenomenon, of bleeding very little, hypomenorrhea.

Prolonged bleeding (metrorrhagia, also meno-metrorrhagia) does not show a clear interval pattern anymore. Dysfunctional uterine bleeding refers to hormonally caused bleeding abnormalities, typically anovulation. All these bleeding abnormalities need medical attention. As pregnant patients may bleed, a pregnancy test forms part of the evaluation of abnormal bleeding.

Estrogens and progesterone-like hormones make up the main active ingredients of birth control pills. Typically they tend to mimic a menstrual cycle in appearance, but to suppress the critical event of the ovulatory cycle, namely ovulation. Normally, a woman takes hormone pills for 21 days, followed by 7 days of non-functional placebo sugar pills or no pills at all; then the cycle starts again. During the 7 placebo days, a withdrawal bleeding occurs; this differs from ordinary menstruation, and skipping the placebos and continuing with the next batch of hormone pills may suppress it. (Two main versions of the pill exist: monophasic and triphasic. With triphasic pills, skipping of the placebos and continuing with the next month's dose can remove the pill's pregnancy protection.) In 2003 the United States Food and Drug Administration (FDA) approved low-dose monophasic birth control pills which induce withdrawal bleedings only every 3 months.

The terms "menstruation" and "menses" come from the Latin mensis (month), which in turn relates to the Greek mene (moon) and to the roots of the English words month and moon — reflecting the fact that the moon also takes close to 28 days to revolve around the Earth. (The synodical lunar month, the period between two new moons, lasts about 29 and a half days).

A whole industry has evolved to cater to menstruating women. Women commonly use sanitary towels (worn outside the vagina) or pads to prevent the soiling of clothes. Disposable ones are made of wood pulp or synthetic products. Throughout history, washable cloths have been used; modern versions are produced made of cotton (often organic), on a parallel to washable diapers or nappies. Tampons (absorbent material inserted into the vagina) have become popular in the West over the last hundred years. A small number of women use reusable rubber menstrual cups.

Pharmaceutical companies also provide products — commonly Non-steroidal anti-inflammatory drugs (NSAIDs) — to relieve menstrual cramps.

Mystics have sometimes elaborated "equivalencies", analogizing the waxing and waning of the moon with influences on human menstruation. In this spiritual, moon goddess, or astrological context some women call menstruation their "moontime". Some ancient views also regarded menstruation as a cleansing of the body: compare bloodletting as a major medical treatment of pre-modern times.

Some religions consider women "unclean" during menstruation.

The Islamic world considers a woman "not in a state to have intercourse" during menstruation. A verse from the Holy Qur'an (with parenthesised interpolations by Dr. Muhammed Muhsin Khan) affirms this: "They ask you concerning menstruation. Say: that is an Adha (a harmful thing for a husband to have sexual intercourse with his wife while she is having her menses), therefore keep away from women during menses and go not unto them till they have purified (from menses and have taken a bath). And when they have purified themselves, then go in unto them as Allâh has ordained for you . Truly, Allâh loves those who turn unto Him in repentance and loves those who purify themselves (by taking a bath and cleaning and washing thoroughly their private parts, bodies, for their prayers, etc.)." (Al-Baqarah 2:222)

See an Islamic review on the subject.

Main article: Niddah.

A ritual exclusion applies to a woman while menstruating and for about a week thereafter, until she immerses herself in a mikvah (ritual bath).

A regular menstrual cycle as described here only occurs in the great apes. Menstrual cycles vary in length from an average of 29 days in orangutans to an average of 37 days in chimpanzees.

Females of other mammalian species go through certain episodes called "estrus" or "heat" in each breeding season. During these times, ovulation occurs and females become receptive to mating, a fact advertised to males in some way. If no fertilization takes place, the uterus reabsorbs the endometrium: no menstrual bleeding occurs. Significant differences exist between the estrus and the menstrual cycle.

• K. Stern and M. K. McClintock: "Regulation of ovulation by human pheromones." Nature, 392 (1998), pages 177 – 179.
• Gangestad et al.: "Women's preferences for male behavioral displays change across the menstrual cycle." Psychological Science, March 2004, vol. 15, no. 3, pages 203 - 207
• Template:Journal reference issue

1. ^ "At what age does a girl get her first period?," from Menstruation and the Menstrual Cycle, National Women's Health Information Center (accessed June 11, 2005).
2. ^ Ibid., "What is a typical menstrual period like?" (accessed June 11, 2005).

• Harry Finley: Online museum of menstruation and women's health, http://mum.org/
• Software for tracking ovulation, natural conception and contraceptions
• Track your likely ovulatory date with this free Ovulation Calendar
• Free Software to watch the menstrual cycle etc. http://www.natuerliche-verhuetung.de/
• Leslie Botha-Williams, Women's Health Educator: A Woman's Guide to Understanding Her Hormone Cycle, http://www.holyhormones.com
• An Islamic answer for the ruling of women menstruating

• Amenorrhoea (or amenorrhea in US).
• Zygote